Add implementation for HTML Character Data methods

The data property is longer exposed as a mutable string. Instead all
manipulations must be performed via the provided API in order to ensure
correct updates for the Document’s attached ranges.

See:
https://dom.spec.whatwg.org/#characterdata

Sources/HTMLCharacterData.m

@@ -8,8 +8,17 @@
 
 #import "HTMLCharacterData.h"
 #import "HTMLNode+Private.h"
+#import "HTMLDocument+Private.h"
+#import "HTMLKitDOMExceptions.h"
+
+@interface HTMLCharacterData ()
+{
+	NSMutableString *_data;
+}
+@end
 
 @implementation HTMLCharacterData
+@synthesize data = _data;
 
 - (instancetype)initWithName:(NSString *)name type:(HTMLNodeType)type data:(NSString *)data
 {
@@ -27,7 +36,7 @@
 
 - (void)setTextContent:(NSString *)textContent
 {
-	[_data setString:textContent ?: @""];
+	[self setData:textContent];
 }
 
 - (NSUInteger)length
@@ -35,12 +44,54 @@
 	return _data.length;
 }
 
+#pragma mark - Data
+
+NS_INLINE void CheckValidOffset(HTMLCharacterData *node, NSUInteger offset, NSString *cmd)
+{
+	if (offset > node.length) {
+		[NSException raise:HTMLKitIndexSizeError
+					format:@"%@: Index Size Error, invalid index %lu for character data node %@.",
+		 cmd, (unsigned long)offset, node];
+	}
+}
+
+- (void)setData:(NSString *)data
+{
+	[self replaceDataInRange:NSMakeRange(0, self.length) withData:data];
+}
+
+- (void)appendData:(NSString *)data
+{
+	[self replaceDataInRange:NSMakeRange(self.length, 0) withData:data];
+}
+
+- (void)insertData:(NSString *)data atOffset:(NSUInteger)offset
+{
+	[self replaceDataInRange:NSMakeRange(offset, 0) withData:data];
+}
+
+- (void)deleteDataInRange:(NSRange)range
+{
+	[self replaceDataInRange:range withData:@""];
+}
+
+- (void)replaceDataInRange:(NSRange)range withData:(NSString *)data
+{
+	CheckValidOffset(self, range.location, NSStringFromSelector(_cmd));
+
+	range.length = MIN(range.length, self.length - range.location);
+
+	[_data replaceCharactersInRange:range withString:data];
+	[self.ownerDocument didRemoveCharacterDataInNode:self atOffset:range.location withLength:range.length];
+	[self.ownerDocument didAddCharacterDataToNode:self atOffset:range.location withLength:data.length];
+}
+
 #pragma mark - NSCopying
 
 - (id)copyWithZone:(NSZone *)zone
 {
 	HTMLCharacterData *copy = [super copyWithZone:zone];
-	copy->_data = self.data;
+	copy->_data = _data;
 	return copy;
 }